The Constitution can be amended and has been repeatedly. But the supremacy of the Constitution is critical because then the government is playing by a set of known rules. You don't need to keep the same rules as they had in 1789, but in my opinion you do need rules (including rules for the government itself, aka a constitution) in order for rule-of-law to function effectively.
Yes, Japanese internment was a clear-cut violation of Constitutional rights. Because constitutional rights were violated, $1.6 billion in reparations were ultimately paid out to internees. Think of it this way: making murder illegal doesn't prevent murder, but it does give a formalized path for recourse when murder occurs. Similarly, having the Constitution specifically spell out what the government can do and cannot do, and how, doesn't prevent government overreach but it does give a formalized path for recourse when government overreach occurs.